The invention concerns a battery operable device having an indicator of the state-of-charge of a battery.
In battery-operated devices, low battery and possible subsequent shut-off of the device are a great source of irritation to the user, especially when the timing of the event cannot be predicted. An indicator accurately showing remaining battery capacity, ie a state-of-charge indicator, enables the user to predict the event and is thus a great benefit to the user. The indicator is especially valuable at the end of the discharge.
The invention is useful for indicating the remaining battery power in battery operable devices, in particular in mobile telephones and laptop computers, which may possibly also be operable from an AC power supply, but also in computer or microprocessor based equipment intended for application where no AC power outlet is available or where, for other reasons, a battery is a preferred power source. The invention may also be useful in other applications where an apparatus is dependent on a battery, such as in an uninterruptible power supply (UPS) where a battery supplies DC power to eg a solid state electronic AC power generator in case of unintended or temporary AC power failure. The battery can be an internal or external battery or an exchangeable battery, and battery operation can be supplemented with the possibility of operation from an AC power source.
1. State-of-the-art
A number of different alternatives exist for state-of-charge indicators.
A first known method involves measurement of the voltage of the battery. On the assumption that the discharge curve of the battery is known, the measured battery voltage is used as an approximate indicator of the discharge state. This method is fairly simple and low cost and requires only very limited information regarding the battery. This method has a limited accuracy since the remaining battery power depends on other factors than the battery voltage alone.
A second known method involves an estimation of used battery capacity or power. The basic principle is that battery capacity is assumed to be known, and by measuring or estimating magnitude and duration of the current drawn from the battery, the remaining battery capacity can be calculated. This method can be implemented in software or in a combination of software and hardware. By implementing a hardware solution, which measures the actual current, accuracy is increased. However, this method too has limited accuracy.
A third known method involves measuring the impedance of a battery. The impedance of the battery changes continuously with the state-of-charge, and an estimation of remaining capacity can be made based on measurements of the change in impedance. The invention offers an improvement to this method, and the known method will therefore be described in the following.
There are two known ways of performing impedance measurements for tracking the change of the impedance over time for use in estimating the state-of-charge of the battery, one being an AC impedance measurement, the other being voltage/current step response. The impedance is defined as the voltage response to unit excitation current, or the ratio of resulting voltage to excitation current.
AC impedance is a complex quantity having two components: a real part and an imaginary part, or magnitude and phase. To measure the AC impedance an alternating current is applied to the battery, and the resulting voltage response is analysed by magnitude and phase or by real and imaginary parts. The AC impedance, whether expressed as real and imaginary parts or as magnitude and phase, depends on frequency. In a known method of estimating the state-of-charge of a battery the AC impedance of the battery is measured and analysed at several frequencies, and the analysis forms the basis for estimating the state-of-charge of a battery. This method requires a frequency generator such as a crystal based clock generator, which generates the necessary clock frequencies for the operation of processor. Frequency dividers and multipliers generate multiples and submultiples of the clock frequency.
In known devices for measuring the AC impedance of a battery the measuring circuits and clock generator are either situated in the battery charger (an xe2x80x9cintelligent chargerxe2x80x9d), which can be connected to and disconnected from the battery, or in the battery (an xe2x80x9cintelligent batteryxe2x80x9d), from where information about the state-of-charge of a battery is sent to the battery operated device. Such devices require that the measuring circuits and clock generator are situated in the device with the sole purpose of providing an indication of the state-of-charge of a battery.
Another known method for analysing the AC impedance is voltage/current step response. A current step is applied, and the voltage response over time is analysed. All the required information about battery impedance can be derived from such impedance measurements. Similar information is obtained and state-of-charge can be estimated. The step response method is normally utilized during charging, where an external charger applies the current pulses.
JP 08179017 discloses a method and a circuit for measuring the AC impedance of a battery. U.S. Pat. No. 5,369,364 discloses a method and an apparatus for measuring the state-of-charge of a lithium-iodine battery in an implantable pacemaker.
2. Problems of the Prior Art
There are problems related to all the known methods referred to above.
Simple measurements of battery voltage alone give very limited accuracy in estimating the state-of-charge of the battery. It is difficult to improve the accuracy without extremely complex models of the discharge curve and influence of temperature, current magnitude and previous life of the battery.
A software-based method for estimating the withdrawn capacity introduces minor errors, which are accumulated over time. Deviation from true battery capacity becomes greater at the end of discharge, which is exactly when it is most important to provide accurate information. Assuming an error of eg 10% in estimations based on current measurements for a device having a standby time of 400 h gives an uncertainty of 40 h (almost two days!) in the estimate of the time of shut-off due to end of battery power. Clearly, this is not acceptable.
The current profiles are also becoming more complex. As the number of current scenarios increase, the possibility of tracking all those scenarios is limited. Current measurements are necessary to get a satisfactory accuracy. Unfortunately, current measurements become tougher as stand-by currents decrease. In addition, with long stand-by, the self-discharge of the battery is also significant. Self-discharge cannot be measured and is very complicated to estimate.
As the battery is cycled, ie charged and discharged, or stored at high temperature, its capacity will be reduced. As this method estimates the remaining capacity, it is necessary to know the true battery capacity at the start of the discharge. Otherwise, predicting when there is no remaining capacity is impossible, regardless of how accurate the estimations of used capacity are. Prediction of battery capacity after storage and cycling requires extensive models, frequent temperature measurements and an intelligent battery that remembers its history.
Known impedance measurements, too, have drawbacks. Introducing an additional chip to the battery pack for AC impedance measurements adds cost and size to the battery pack. As the chip must have an internal clock, a clock crystal is added to the battery pack. For large portable applications, this may not be a great problem. However, in small applications where size and cost are major issues, the separate measurement chip in the battery pack is inconvenient.
It is difficult to use this method to continuously monitor the state-of-charge. During stand-by, most functions should be in low power mode and it is not feasible to continuously perform measurements of battery impedance. Every measurement and calculation consumes energy.
Analysis of the response to an applied current or voltage step is difficult to perform in a mobile telephone. As discussed above in connection with AC impedance measurement, the important information is in the frequency region from 1 Hz to 10 kHz. This means that the duration of an applied voltage/current step would have to be in the range of seconds. This is difficult to achieve in a mobile telephone. Shorter duration is not useful.
The invention uses a simplified AC impedance measurement as a basis for estimations of remaining battery power.
The invention uses a cost reduced AC impedance measurement. Measurements are moved from the battery pack to the telephone. Clock frequencies are already available from clock crystals in the telephone. Consequently, the telephone will use frequencies derived from the clock crystals already existing in the telephone, such as the 32 kHz clock crystal or the 13 MHz clock crystal. The frequencies derived from the existing clock crystals are possibly not the frequencies that give the most accurate impedance measurements. However, by using the components already exiting in a telephone, cost savings can be achieved.
The invention provides a simple way to determine the battery state-of-charge in portable devices, in particular in small portable devices such as mobile telephones. The invention makes use of a clock frequency generator existing in the device. It can either be used separately or together with another method for estimating the state-of-charge of the battery, eg calculations of used capacity. This has a certain merit, in particular if the accuracies of the individual methods are not satisfactory. By combining two low cost methods the total accuracy of the system can be raised significantly.
Cost is lower compared to a dedicated chip in the battery pack
The solution is more compact than a dedicated chip in the battery pack,
The solution does not introduce any impedance in the current path to the power amplifier.
The invention can help the software-based estimations to handle self-discharge and ageing of the battery.